Photoelectric sensors, also known as optoelectronic sensors, are used for detecting objects. With so-called disposable sensors a light beam sent by an emitter is received by a receiver and retransmitted for evaluation as an electric signal.
An optoelectronic sensor for detecting objects with a sending block for sending a light signal and a receiving block for receiving the light signal and generating a received signal from the received light signal is known from DE 10 2011 000 857. The light signal is modulated with a code sequence, wherein a correlation signal is formed from the received signal and the code sequence used in a correlation unit. The correlation unit receives the code sequence. Alternatively an optical synchronisation of the sending block and the receiving block not described in detail will take place.
A photoelectric switching means for detecting an object in the path of the beam between a light source and a photoelectric converter is described in DE 311 88 38 C2. The switching means comprises a light source part, a light receiving part, a signal processing part and an auxiliary part for controlling and synchronizing the operating mode of the light source part and the light receiving part. The light receiving part comprises the photoelectric converter, which converts light falling onto the same into an electric signal. In order to process input data based on the electric signal the signal processing part is equipped with pulse shaper. When a switching means with separately arranged light source and light receiver is used, the light source part is connected with a separate oscillator. The frequency of this oscillator is a little higher than that of an oscillator whose output signal is used as a clock signal for entering input data. The frequency of this oscillator is synchronized with the frequency of the separate oscillator.
Even very short-term interference of the light beam, for example due to external light or through covering, can lead to an interruption of synchronisation with such switching means. Synchronisation is however essential for the use of pulsed light beams. It enables the determination of measurement times within the pulses.